Dawn G. Flicker

Beryllium liner implosion experiments on the Z accelerator in preparation for magnetized liner inertial fusion

Multiple experimental campaigns have been executed to study the implosions of initially solid beryllium (Be) liners (tubes) on the Z pulsed-power accelerator. The implosions were driven by current pulses that rose from 0 to 20 MA in either 100 or 200 ns (200 ns for pulse shaping experiments). These studies were conducted in support of the recently proposed Magnetized Liner Inertial Fusion concept [Slutz et al., Phys. Plasmas 17, 056303 (2010)], as well as for exploring novel equation-of-state measurement techniques. The experiments used thick-walled liners that had an aspect ratio (initial outer radius divided by initial wall thickness) of either 3.2, 4, or 6. From these studies, we present three new primary results. First, we present radiographic images of imploding Be liners, where each liner contained a thin aluminum sleeve for enhancing the contrast and visibility of the liners inner surface in the images. These images allow us to assess the stability of the liners inner surface more accurately and more directly than was previously possible. Second, we present radiographic images taken early in the implosion (prior to any motion of the liners inner surface) of a shockwave propagating radially inward through the liner wall. Radial mass density profiles from these shock compression experiments are contrasted with profiles from experiments where the Z accelerators pulse shaping capabilities were used to achieve shockless (“quasi-isentropic”) liner compression. Third, we present “micro-B” measurements of azimuthal magnetic field penetration into the initially vacuum-filled interior of a shocked liner. Our measurements and simulations reveal that the penetration commences shortly after the shockwave breaks out from the liners inner surface. The field then accelerates this low-density “precursor” plasma to the axis of symmetry.

Solid liner implosions on Z for producing multi-megabar, shockless compressions

Current pulse shaping techniques, originally developed for planar dynamic material experiments on the Z-machine [M. K. Matzen et al., Phys. Plasmas 12, 055503 (2005)], are adapted to the design of controlled cylindrical liner implosions. By driving these targets with a current pulse shape that prevents shock formation inside the liner, shock heating is avoided along with the corresponding decrease in electrical conductivity ahead of the magnetic diffusion wave penetrating the liner. This results in an imploding liner with a significant amount of its mass in the solid phase and at multi-megabar pressures. Pressures in the solid region of a shaped pulse driven beryllium liner fielded on the Z-machine are inferred to 5.5 Mbar, while simulations suggest implosion velocities greater than 50kms-1. These solid liner experiments are diagnosed with multi-frame monochromatic x-ray backlighting which is used to infer the material density and pressure. This work has led to a new platform on the Z-machine that can be used to perform off-Hugoniot measurements at higher pressures than are accessible through magnetically driven planar geometries.Current pulse shaping techniques, originally developed for planar dynamic material experiments on the Z-machine [M. K. Matzen et al., Phys. Plasmas 12, 055503 (2005)], are adapted to the design of controlled cylindrical liner implosions. By driving these targets with a current pulse shape that prevents shock formation inside the liner, shock heating is avoided along with the corresponding decrease in electrical conductivity ahead of the magnetic diffusion wave penetrating the liner. This results in an imploding liner with a significant amount of its mass in the solid phase and at multi-megabar pressures. Pressures in the solid region of a shaped pulse driven beryllium liner fielded on the Z-machine are inferred to 5.5 Mbar, while simulations suggest implosion velocities greater than 50kms-1. These solid liner experiments are diagnosed with multi-frame monochromatic x-ray backlighting which is used to infer the material density and pressure. This work has led to a new platform on the Z-machine that can be ...

Determination of pressure and density of shocklessly compressed beryllium from x-ray radiography of a magnetically driven cylindrical liner implosion.

We describe a technique for measuring the pressure and density of a metallic solid, shocklessly compressed to multi-megabar pressure, through x-ray radiography of a magnetically driven, cylindrical liner implosion. Shockless compression of the liner produces material states that correspond approximately to the principal compression isentrope (quasi-isentrope). This technique is used to determine the principal quasi-isentrope of solid beryllium to a peak pressure of 2.4 Mbar from x-ray images of a high current (20 MA), fast (~100 ns) liner implosion.

Ethane-Xenon Mixtures under Shock Conditions.

Mixtures of light elements with heavy elements are important in inertial confinement fusion. We explore the physics of molecular scale mixing through a validation study of equation of state (EOS) properties. Density functional theory molecular dynamics (DFT-MD) at elevated temperature and pressure is used to obtain the thermodynamic state properties of pure xenon, ethane, and various compressed mixture compositions along their principal Hugoniots. In order to validate these simulations, we have performed shock compression experiments using the Sandia Z-Machine. A bond tracking analysis correlates the sharp rise in the Hugoniot curve with the completion of dissociation in ethane. Furthermore, the DFT-based simulation results compare well with the experimental data along the principal Hugoniots and are used to provide insight into the dissociation and temperature along the Hugoniots as a function of mixture composition. Interestingly, we find that the compression ratio for complete dissociation is similar for several compositions suggesting a limiting compression for C-C bonded systems.

Mesoscale simulation of shocked poly-(4-methyl-1-pentene) (PMP) foams.

Hydrocarbon foams are commonly used in high energy-density physics (HEDP) applications, for example as tamper and ablation materials for dynamic materials or inertial confinement fusion (ICF) experiments, and as such are subject to shock compression from tens to hundreds of GPa. Modeling of macro-molecular materials like hydrocarbon foams is challenging due to the heterogeneous character of the polymers and the complexity of voids and large-scale structure. Under shock conditions, these factors contribute to a relatively larger uncertainty of the post-shock state compared to that encountered for homogenous materials; therefore a quantitative understanding of foams under strong dynamic compression is sought. We use Sandias ALEGRA-MHD code to simulate 3D mesoscale models of poly-(4-methyl-1-pentene) (PMP) foams. We devise models of the initial polymer-void structure of the foam and analyze the statistical properties of the initial and shocked states. We compare the simulations to multi-Mbar shock experimen...

Equation of state of argon : experiments on Z, density functional theory (DFT) simulations, and wide-range model.

Over the last few years, first-principles simulations in combination with increasingly accurate shock experiments at multi-Mbar pressure have yielded important insights into how matter behaves under shock loading. Noble gases like argon are particularly interesting to study as a model system due to the closed shell electronic structure that results in a weak interatomic interaction at normal conditions followed by pronounced ionization and strong interaction under compression. Cryogenic argon is also optically transparent while shocked argon is metallic, displaying a reflective shock front, thus allowing for shock velocity measurement to very high precision. In this report, we present experimental results for shock compression of liquid cryogenic argon to several Mbar using magnetically accelerated flyers on the Z machine, first-principles simulations based on Density Functional Theory, and an analysis of tabular equations of state (EOS) for argon, including a newly developed wide-range EOS model.

Particle-in-cell and hypernetted chain models of two-component, two-temperature coupled classical plasmas

Three-dimensional simulations of moderately to strongly coupled electron-ion and multicomponent classical plasmas using the particle-in-cell method are presented. The simulations resolve sub-Debye-length interparticle spacing to accurately model the dynamics of these systems. We consider realistic mass ratios and quasiequilibrium conditions with different component temperatures which are relevant on short time scales. The simulation results are in very good agreement with classical hypernetted chain calculations for dense electron-ion and ion-ion plasmas. Our results demonstrate the feasibility and utility of large-scale particle-in-cell simulations for the modeling and analysis of multicomponent moderately and strongly coupled plasmas.

Simulations of Dynamic Laser/Plasma X-Ray Production

Intense laser beams focused onto thin high-atomic-number targets can generate short intense bursts of MeV X-rays from a small area of the target. Such systems are being developed as short-pulse point-projection X-ray sources for imaging high-density objects. Here, large-scale (400-million macroparticles and 15-million grid cells) 3-D particle-in-cell simulations are described that model the dynamic interaction between the laser beam, a blowoff plasma layer, and the solid-density target. The simulations self-consistently treat the nonlinear interaction between the incident laser pulse and the blowoff plasma layer where a relativistic electron beam is generated. This beam propagates into the solid-density high-atomic-number target where MeV bremsstrahlung is generated. The model tracks the generation, propagation, and self-absorption of radiation in the blowoff plasma, target, and beyond. Radiation production (fluence and energy spectrum) is characterized in the simulations as a function transverse target size, laser-injection angle, and laser energy. The simulated X-ray fluence for the case of a 45 °-angle-of-incidence 100-J 0.5-ps laser pulse with a 6- μm FWHM focus produces a peak dose in excess of 0.2 rad from a 10-μm-thick square gold target, consistent with experimental measurements.

ALEGRA-HEDP simulations of the dense plasma focus.

We have carried out 2D simulations of three dense plasma focus (DPF) devices using the ALEGRA-HEDP code and validated the results against experiments. The three devices included two Mather-type machines described by Bernard et. al. and the Tallboy device currently in operation at NSTec in North Las Vegas. We present simulation results and compare to detailed plasma measurements for one Bernard device and to current and neutron yields for all three. We also describe a new ALEGRA capability to import data from particle-in-cell calculations of initial gas breakdown, which will allow the first ever simulations of DPF operation from the beginning of the voltage discharge to the pinch phase for arbitrary operating conditions and without assumptions about the early sheath structure. The next step in understanding DPF pinch physics must be three-dimensional modeling of conditions going into the pinch, and we have just launched our first 3D simulation of the best-diagnosed Bernard device.

Conceptual designs of 300-TW and 800-TW pulsed-power accelerators

We have developed conceptual designs of two next-generation petawatt-class pulsed-power accelerators. The designs are based on the architecture described in Ref. [1]. The prime power source of both designs is a system of lineartransformer drivers (LTDs) [2,3]. Both designs use six water-insulated radial-transmission-line impedance transformers [1,4,5] to transport the power generated by the LTDs to a six-level vacuum-insulator stack. The stack is connected to six radial magnetically insulated transmission lines (MITLs); the MITLs are joined in parallel at small radius by a triple-post-hole vacuum convolute [6-9]. The convolute delivers the combined power of the six MITLs to a single short MITL that transmits the power to the load. The first accelerator will generate a peak electrical power of 300 TW, and deliver an effective peak current of 50 MA to a z pinch that implodes in 130 ns. This accelerator is 35 m in diameter, and will fit within the existing Z-accelerator building. The second, which is 52 m in diameter, will generate 800 TW, and deliver an effective peak current of 66 MA to a pinch that implodes in 120 ns. Both accelerators will allow high-energy-density physics experiments to be conducted over heretofore inaccessible parameter regimes.